Growth stresses and their consequences on tree mechanics
Résumé
During tree growth, mechanical stresses accumulate in the trunk and branches due to the increasing
weight and wood maturation. The progressive increase of the weight of the tree induces self-support
stress whose distribution is very different from a homogeneous beam with load uniformly distributed
over its section. Because the vertical position is challenging gravity, increase in tree weight during
growth may disturb the mechanical equilibrium of the stems or branches. During the formation of a
new wood layer at the periphery of the stem or branch, physico-chemical transformations occurring
with the thickening of the cell wall generate mechanical stresses at periphery, called maturation
stress, and used by the tree to control posture and preserve shape in a standing position. The
combination of self-support stress and maturation stress is called growth stresses.
Measuring the corresponding released strain at the periphery of stems may evidence maturation
stress: isolating a piece of stressed wood from surrounding tissues leads to shrinkage if the stress is
tensile, swelling if the stress is compressive. These measurements may be achieved by different
experimental means, using simple strain gauges for example. The occurrence of radial cracks at a log
end, or lumber crooking, may also evidence growth stresses. We will present methods to evaluate
these growth strains.
Patterns of growth stress always exhibit very non-linear variations across the diameter: although the
stress increment associated to a radius increment has linear variations within the section, the
integration of this stress over growth is non-linear. We will present the modelling of some growth
scenarios leading to very different growth patterns, and discuss the consequences of the state of prestressing
on stem strength: What effect of pre-stressing on tree buckling? What happens in case of
wind loading? May a branch break due to its self-loading?